Preprints
https://doi.org/10.5194/acp-2023-28
https://doi.org/10.5194/acp-2023-28
25 Jan 2023
 | 25 Jan 2023
Status: a revised version of this preprint was accepted for the journal ACP and is expected to appear here in due course.

Particle shapes and infrared extinction spectra of nitric acid dihydrate crystals: Optical constants of the β-NAD modification

Robert Wagner, Alexander D. James, Victoria L. Frankland, Ottmar Möhler, Benjamin J. Murray, John M. C. Plane, Harald Saathoff, Ralf Weigel, and Martin Schnaiter

Abstract. Satellite- and aircraft-based mid-infrared measurements of polar stratospheric clouds (PSCs) have provided spectroscopic evidence for the presence of β-NAT (nitric acid trihydrate) particles. Metastable nitric acid hydrate phases such as α-NAT and α-NAD (nitric acid dihydrate) have been frequently observed in laboratory experiments, but not yet detected as a constituent of PSCs in atmospheric measurements. As for the β-NAD modification, its formation was first observed in X-ray diffraction measurements when the low-temperature α-NAD phase was warmed to a temperature above 210 K. Its infrared spectrum has been reported, but so far no optical constants have been derived that could be used as input for infrared retrievals of PSC composition. In this work, we show that β-NAD particles were efficiently formed in isothermal, heterogeneous crystallisation experiments at 190 K from supercooled HNO3/H2O solution droplets containing an embedded mineral dust or meteoric smoke particle analogue. An inversion algorithm based on a T-matrix optical model was used to derive for the first time the mid-infrared complex refractive indices of the β-NAD modification from the measured extinction spectrum of the particles. In contrast to the heterogeneous crystallisation experiments, the α-NAD phase was formed when the HNO3/H2O solution droplets did not contain a solid nucleus and crystallised homogeneously. Using a light scattering detector that recorded two-dimensional scattering patterns of the crystallised NAD particles, we were able to determine predominant shapes of the α- and β-NAD crystals. We found that α-NAD grew into elongated, needle-shaped crystals, while β-NAD particles were compact in shape. This agrees with previously reported images of α- and β-NAD particles grown on the cryo-stage of an Environmental Scanning Electron Microscope.

Robert Wagner et al.

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse

Robert Wagner et al.

Robert Wagner et al.

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Polar Stratospheric Clouds (PSCs) catalyze ozone depletion at high latitudes with important impacts on energetic, biologically effective UV radiation. The present study is a unique experiment into PSC particle optical properties and formation performed in a large, coolable cloud chamber. Robert Wagner and his colleagues find evidence that a nitric acid dihydrate can be efficiently crystallized in PSCs through heterogeneous nucleation, for example by mineral dust or micrometeorites. It has a "beta-polymorph" crystalline structure - with distinct optical properties - clearly separable from an alpha-polymorph that forms at low temperatures through homogeneous nucleation. Interestingly, electron microscopy, infrared spectroscopy, X-ray diffraction and calculations all point to the same conclusion. The results have important implications for our understanding and the detection of PSCs.
Short summary
Polar stratospheric clouds (PSCs) play an important role in the depletion of stratospheric ozone. They can consist of different chemical species, including crystalline nitric acid hydrates. Infrared measurements are an important technique for determining the composition, shape and phase of PSC particles. For the quantitative analysis of infrared spectra, we have for the first time determined accurate optical constants and predominant particle shapes of nitric acid dihydrate in its β phase.
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